P-Glycoprotein (P-gp) is a member of the large ATP binding cassette super family of transport proteins also called traffic ATPases. P-gp is composed of two homologous halves, each containing six transmembrane domains, separated by flexible linker polypeptides. P-gp is a 170 kDa membrane bound protein that effluxes a wide variety of structurally unrelated drugs out of cells. This efflux pump is responsible for export of wide variety of hydrophobic natural products, drugs, and linear and cyclic peptides from the cytoplasm and cytoplasmic membrane of eukaryotic cells, using energy from ATP hydrolysis.
Various mechanisms have been proposed to explain the efflux of drugs and various xenobiotics by this transporter. However, there are three prevalent models, the hydrophobic vacuum model (HVC), the pore model, and the flippase model that have been used to explain the mechanism by which P-gp effluxes out drugs. The hydrophobic vacuum model is the most widely accepted model of P-gp action. According to the HVC model P-gp acts as a vacuum cleaner, moving compounds that are substrates from the lipid bilayer into the extra cellular space.
P-gp is ubiquitously expressed on human tissues such as intestinal mucosa, brain capillary endothelial cells, biliary canaliculus, and kidney tubules. Thus, the ubiquitous presence of this transporter and the broad substrate specificity of P-gp makes it a major factor responsible for sub-therapeutic levels of various drugs in the blood and various tissues. Recently, it has been reported that presence of this transporter on the brush border membrane of the intestinal epithelium not only decreases the permeability of various bioactive agents but also increases the metabolism of various drugs by back effluxing the drugs and increasing the exposure time of the drug in the cells as well as in the lumen.
Thus, bioavailability of various anticancer drugs, anti-HIV drugs, calcium channel drugs, and other drugs which are P-gp substrates is limited by this efflux transporter. Over expression of P-gp by tumor cells confers multidrug resistance. Efflux of many anticancer drugs including taxol, vincristine, vinblastine, actinomycin D, colchicines, and daunorubicin, from tumor cells makes P-gp a major barrier to chemotherapy. High expression of this transporter on the blood-brain-barrier (BBB) restricts the entry into the brain of P-gp substrates such as anti-HIV drugs such as ritonavir, saquinavir, nelfinavir, and various anticancer drugs, and thus imposes a major challenge in the treatment of various diseases of the brain.
Expression of this efflux transporter on various body tissues and cells not only influences the in vivo disposition of various therapeutically active drugs but also greatly influences pharmacokinetics of the drug. It has been known that inhibition of P-gp by various modulators can lead to improved bioavailability of drugs across the intestines, the kidneys, and the BBB. Various modulators that are inhibitors of P-gp are often co-administered so that they can inhibit P-gp and increase bioavailability when given simultaneously with other bioactive agents. However, usage of these compounds is limited by their toxicity, owing to the high serum concentration that is achieved with the doses required to inhibit P-gp. Thus, although various approaches have been studied to overcome P-gp mediated drug efflux, P-gp remains a major barrier to bioavailability, chemotherapy, and effective permeation of P-gp substrates into the brain and other tissues.
Besides efflux transporters, such as P-gp, there are various membrane transporters/receptors, such as peptide or vitamin transporters, that help in the influx of various nutrients and other compounds that are substrates for these transporters into various organs, cells, and across various barriers, including the blood brain barrier.
Thus, there exists a need for bioactive compounds that are not recognized by P-gp as a substrate, but that are substrates of membrane transporters/receptors. There is also a need for increasing the bioavailability of bioactive compounds that are P-gp substrates. Further, there is a need to increase the concentration of bioactive compounds that are P-gp substrates in sanctuary sites of a mammalian subject. Still further, there is a need to enhance delivery to cells of bioactive compounds that are P-gp substrates.